1. Field of the Invention
The present invention relates to a fabrication method of a semiconductor device, and particularly to a semiconductor device having transistors with different high voltage gates as well as high voltage drains and an MNOS memory transistor within the same semiconductor layer.
2. Related Art
In the fabrication process of a high voltage transistor, by comparison to a low voltage drive transistor, a high temperature process for forming a deep well and a thick gate insulating layer are necessary. This high temperature process is unique to a forming process of the high voltage drive transistor, and, typically, the high voltage transistor for high voltage operation and the low voltage drive transistor were individually formed.
On the other hand, research and development on the so-called SOC (System On Chip) has been conducted in recent years to combine a plurality of ICs on one piece of IC chip.